Distribution patterns of macroinvertebrates in vegetated, shallow, soft-bottom bays of the Baltic Sea. Licentiate thesis.

Transkript

1 Distribution patterns of macroinvertebrates in vegetated, shallow, soft-bottom bays of the Baltic Sea Licentiate thesis by Joakim Hansen Supervisors: Lena Kautsky and Sofia Wikström Plants & Ecology Plant Ecology 2007/8 Department of Botany Stockholm University

2 Plants & Ecology Plant Ecology Department of Botany Stockholm University S Stockholm Sweden Plant Ecology ISSN Printed by Solna Printcenter Cover: The snail Bithynia tentaculata crawling on the macrophyte Potamogeton pectinatus, with Chara baltica in the background. Photo by Joakim Hansen.

3 Summary The present thesis identifies distribution patterns of macroinvertebrates in shallow, land-uplift bays of the Baltic Sea, and relates these to abiotic and biotic environmental factors. This was conducted on two scales: (a) between bays which differ in abiotic constrains mainly due to isolation from the sea; and (b) between macrophyte habitats within these bays. The relationship between the macroinvertebrate community and the morphometric isolation of the bays was tested using water exchange as a proxy for isolation. The specific role of macrophyte taxon diversity as an indicator of the macroinvertebrate community was also tested. Additionally, the active habitat preference by four invertebrate taxa for three macrophyte species was investigated. The results showed that the biomass and taxon richness of macroinvertebrates decrease, and the taxon composition changes, with the natural isolation of bays from the sea. Water exchange and macrophyte taxon richness explained about 80% of the variation in univariate measures of the fauna community. The results indicated a hierarchical relationship between water exchange, macrophyte taxon richness and univariate measures of the fauna community; decreased biomass and taxon richness of fauna were related to decreased macrophyte taxon richness, which in turn was related to decreased water exchange. In multivariate analyses, water exchange and macrophyte taxon richness were found to explain about 50% of the variation in the fauna community. The community changed from a dominance of gastropods and bivalves in open bays to a dominance of mainly a few insect taxa in isolated bays. Results from the habitat preference experiment revealed an active habitat choice by three of the four studied invertebrates. Two crustacean genera, Idotea and Gammarus, preferred the macrophyte Myriophyllum spicatum over Potamogeton pectinatus and Chara baltica. The gastropod Theodoxus fluviatilis had a tendency of preferring P. pectinatus. My results suggest that: (a) with the isolation of bays from the sea, macroinvertebrate density and species diversity decreases and change in composition; and (b) the distribution of macroinvertebrates differs between patches of different vegetation types growing under similar abiotic conditions within bays. The present findings also suggest that macrophyte taxon richness can serve as an indicator for the fauna community. It is also clear, however, that more research is needed before applying the findings in any generalized way. 3

4 Sammanfattning Den här avhandlingen identifierar fördelningsmönster av ryggradslösa djur i grunda vikar i Östersjön. Studien behandlar djur- och växtsamhällen i havsvikar med mjuka sedimentbottnar, vilka förändras i form och sakta isoleras från havet genom den naturliga landhöjningen i Nordeuropa. Fördelningsmönstren av djur relateras till olika miljöfaktorer. Studien är utförd på två olika skalor: (a) mellan vikar som skiljer sig i djup, form och salthalt, främst pga. isolering från havet; och (b) mellan olika vattenväxter inom dessa vikar. Sambandet mellan djursamhället och isolationen av vikar från havet testades med vattenutbyte som ett mått på isoleringsgrad. Vidare undersöktes om artantalet av vattenväxter i vikarna kan fungera som en indikator för skillnader i djursamhället mellan vikarna. Utöver detta studerades om fyra olika djur uppvisade något aktivt val mellan tre olika vattenväxter som livsmiljö. Resultaten visade att biomassan och artantalet ryggradslösa djur minskar med den naturliga isoleringen av vikar från havet. Vattenutbyte och antal växtarter visade sig förklara ca 80 % av variationen i djurbiomassa och antal djurarter i vikarna. Resultaten indikerar även ett hierarkiskt samband mellan vattenutbyte, antal växtarter samt biomassa och artantal av djur. Minskad biomassa och artantal av djur hade ett direkt samband med minskat antal växtarter, som i sin tur var relaterat till minskat vattenutbyte i vikarna. I analyser av djursamhället med hänsyn till varje art visade sig vattenutbyte och antalet växtarter förklara ca 50 % av variationen i artsammansättningen. Djursamhället förändrades från en dominans av snäckor och musslor i de mest öppna vikarna till en dominans av främst några få insekter i de mest isolerade vikarna. Resultaten från studien av djurens val av livsmiljö visade att tre av de fyra undersökta djuren uppvisade ett aktivt val mellan vattenväxterna. Preferensen skiljde sig dock mellan djuren. Två kräftdjur, tånggråsuggor (Idotea spp.) och märlkräftor (Gammarus spp.), föredrog växten axslinga (Myriophyllum spicatum) framför borstnate (Potamogeton pectinatus) och grönsträfse (Chara baltica). I motsats till dem visade båtsnäcka (Theodoxus fluviatilis) en tendens att föredra borstnate framför de två andra vattenväxterna. Resultaten visar att: (a) med ökad isolering av vikar från havet minskar djursamhället i artantal och densitet, samt förändras i sammansättning; och (b) fördelningen av djur skiljer sig mellan olika vattenväxter som växer under liknande miljöförhållanden i dessa vikar. Vidare föreslås att artantal av undervattensväxter kan användas som en indikator för skillnader i djursamhället mellan grunda mjukbottenvikar i Östersjön. Fler undersökningar bör dock göras innan dessa slutsatser tillämpas i större omfattning. 4

5 List of papers The thesis is based on the following papers, referred to by their roman numerals: I. Hansen, J. P., Wikström, S. A. and Kautsky, L. Effects of water exchange and vegetation on the macroinvertebrate fauna composition of shallow land-uplift bays in the Baltic Sea. Manuscript, submitted to Estuarine, Coastal and Shelf Science II. Hansen, J. P., Axemar, H., Wikström, S. A. and Kautsky, L. Habitat preference of plant-associated macroinvertebrates in brackish shallow soft-bottom bays. Manuscript 5

6 Introduction Common soft-bottom habitats along the Baltic Sea coastline are shallow, enclosed bays with abundant macrophyte vegetation. These wave-protected habitats are naturally nutrient-rich and compared to more open coastal habitats they have a rapid warming of the water volume during spring. These features results in a productive habitat with a rich macrophyte community (Munsterhjelm 1997). The unique characters of the shallow bays is also crucial for the reproduction of many fish species (Karås & Hudd 1993, Karås 1999). Benthic communities in the Baltic Sea are significantly affected by variations in the abiotic conditions. The flora and fauna communities of both hard and soft bottoms are strongly influenced by factors such as light, salinity, substrate, nutrient levels, water movements, depth, temperature and ice cover (e.g. Kautsky 1988, Uitto & Sarvala 1990, Schramm 1996, Snoeijs 1999, Boström et al. 2006). With the latitudinal gradient in the Baltic Sea the salinity, temperature and growing season duration decreases from south to north. Additionally, the coastal areas of the Baltic Sea undergo a geomorphometric evolution with the natural isostatic land-uplift process. In the morphometric development, open bays and sounds, for example, are gradually isolated from the sea forming bays with narrow openings, which finally are completely cut off from the sea (Munsterhjelm 1997). This geomorphological isolation gradient is also a very important factor structuring the macrophyte community in the coastal bays as it changes the hydrographical conditions and factors such as water depth and movements decreases (Munsterhjelm 1997, Appelgren & Mattila 2005). One of the most important anthropogenic factors inducing changes in the benthic habitats of the Baltic Sea is nutrient enrichment resulting in a set of eutrophication effects (e.g. Cederwall & Elmgren 1990, Kautsky 1991, Schramm 1996, Bonsdorff et al. 1997). With eutrophication there has been a general decline of slow-growing and perennial species and an increase of fast-growing, mainly annual benthic and planktonic algal species, but also some rooted macrophytes. In sheltered soft-bottom bays of the Baltic Sea charophytes have generally declined and been replaced by angiosperms more tolerant to low light conditions (Blindow 1992, Blindow 2000, Schubert & Blindow 2003, Munsterhjelm 2005). These bays have also been suggested to shift from a macrophyte to a phytoplankton dominated state under very high nutrient loadings (Dahlgren & Kautsky 2004). Increased dredging and boat traffic are other factors negatively affecting the macrophyte community in the shallow bays 6

7 by mechanical impact, altered hydrographical conditions and increased turbidity (Eriksson et al. 2004). Since the late 1990s there has been an ongoing process of mapping, describing, and evaluating the shallow coastal soft-bottom areas of the Baltic Sea for conservation and management purposes (e.g. Wallström & Persson 1997, Andersson 2000, Johansson & Persson 2005). These efforts have largely concerned regional management plans focusing on the submerged macrophyte vegetation and fish reproduction. The macrophyte community is used as an indicator of ecological status for protection of these habitats (e.g. Wallström & Persson 1997, Andersson 2000, Kautsky et al. 2004, Johansson & Persson 2005), but knowledge about the relationship between the macrophyte vegetation and other biotic units in the ecosystem is poor. Accordingly there is a need to study the structure and function of the ecosystem in more detail and to develop indicator systems which helps in the monitoring of this habitat. This applies specifically to the macroinvertebrate fauna which is one of the least studied units of the food web. The role of macrophytes for fauna communities in aquatic soft-bottom habitats has been recognised for long (e.g. Pond 1905, Moore 1915, Klugh 1926, Krecker 1939). On soft bottoms macrophytes often constitute the only three dimensional structure in an otherwise flat seascape. They are important for foraging, predator-avoidance and/or as a direct food resource for macroinvertebrates. Generally, macroinvertebrate densities and species richness are considerably higher where plant structures are provided, and additionally increase with increasing plant shoot density, biomass, leaf area or morphological complexity of macrophyte habitats (Orth et al. 1984, Diehl & Kornijóv 1998, Hemminga & Duarte 2000, Taniguchi et al. 2003, McAbendroth et al. 2005). The taxonomic diversity of macroinvertebrates has also been observed to increase with increased number of macrophyte taxa in a habitat (Brown et al. 1988, Parker et al. 2001, Bazzanti et al. 2003). The increased density and diversity of macroinvertebrates in highly structured and species diverse macrophyte habitats can be explained by increased availability of microhabitats that e.g. reduce predation pressure and increased availability of food. In this thesis I identify distribution patterns of macroinvertebrates in shallow soft bottom bays of the Baltic Sea, and relate these to abiotic and biotic environmental factors. This is conducted on two different scales; (a) between bays which differ in abiotic constrains mainly 7

8 due to isolation from the sea (Paper I) and (b) between macrophyte habitats within these bays (Paper II). The thesis further gives insight in the role of plant habitat variability (species diversity and structural complexity) for general fauna distributions. At present, I am not aware of any previous studies that have both described the macrophyte-associated invertebrate fauna in shallow soft-bottom bays of the Baltic Sea and also identified factors structuring these communities. Aims of the thesis The aims of the thesis were to: 1) relate differences in the macroinvertebrate fauna community of shallow soft-bottom bays in the Baltic Sea to the geomorphometric isolation of bays from the sea (Paper I). 2) study the specific role of the submerged macrophytes as an indicator of the macroinvertebrate fauna community in these bays (Paper I). 3) investigate if macroinvertebrates display any active habitat preference between submerged macrophytes in shallow soft-bottom bays in the Baltic Sea (Paper II). 4) study the specific role of habitat variability (structural complexity and species diversity) on habitat preferences of single macroinvertebrate taxa (Paper II). 8

9 Materials & Methods Study areas and habitat description The study was conducted in the northern Baltic Proper and the southern Bothnian Sea. Benthic macrophytes and macroinvertebrates were sampled in 12 shallow soft-bottom bays in four different regions within this latitudinal gradient (Paper I; Fig. 1). Samples were collected in late July and early August 2003 and The sampled bays were chosen to include a topographic isolation gradient from very open to enclosed isolated bays. Experiments were also conducted at the Askö laboratory in July 2006, where macrophytes and macroinvertebrates were collected from two shallow soft-bottom bays (Paper II; Fig 1). All bays included in the study were subjected to a very limited degree of anthropogenic disturbance (e.g. from agriculture, housing, and jetties). Shallow coastal areas of the Baltic Sea undergo a geomorphometric evolution with the natural isostatic land-uplift process, which has proceeded since the last ice age approximately 10 thousand years ago. In the process open bays and sounds (juvenile flads) decrease in depth and are successively isolated from the sea, forming bays with narrow openings, often with shallow thresholds (flads and gloes; Fig. 2; Munsterhjelm 1997). The development time from juvenile flads to gloes depends on factors such as the land uplift rate (which varies with latitude), local topography, drainage area, bay area and ice cover (Munsterhjelm 1997). The process can be rapid within a century or take centuries. With the geomorphological isolation of the bays the hydrographical characteristics change, e.g. wave exposure, water exchange and water circulation decrease. These changes are reflected in the succession of aquatic macrophytes, in which the dominant form of vegetation changes with isolation stage, as described by Munsterhjelm (1997) and tested by Appelgren & Mattila (2005) for the northern Baltic Proper. In early morphometric development stages the flora is composed of relative high proportions of hard-bottom algae with Fucus vesiculosus L. being most dominant, often forming loose-laying mats on the bottom (pers. obs.). In later development stages freshwater angiosperms such as Potamogeton perfoliatus L., Potamogeton pectinatus L. and Myriophyllum spicatum L. dominate together with charophytes (Fig. 2). In very isolated bays Chara species become more dominant together with the angiosperm Najas marina. The macrophyte taxon richness decrease with the succession from taxa in open areas to 1-8 taxa in the most isolated bays (Munsterhjelm 1997). The vegetative succession is however not 9

10 strictly consistent with the morphometric development of the bays, and there are often gradients within the bays, and from the outer to the inner archipelago areas, due to differences in water circulation, wave exposure and bottom material (Munsterhjelm 1997). As for the macrophytes, young-of-the-year fish assemblages also differ between different morphometric development stages of bays in the northern Baltic Proper (M. Snickars & A. Sandström pers. comm.). The fish assemblages changes from a stickleback-dominated community in the most open bays, via a perch-dominated community, to a cyprinid-dominated community in the most sheltered bays. BB 1 2 BS 3 4 BP Fig. 1. Map of the Baltic Sea with sampled regions (open circles and numbers). The filled square mark location of the Askö Laboratory. Abbreviation of sea areas: BB = Bothnian bay, BS = Bothnian Sea, BP = Baltic Proper. (Map modified from Einstein, 2006). 10

11 I. 5 m Loose laying II. III. IV. Fucus vesiculosus Potamogeton perfoliatus Potamogeton pectinatus Myriophyllum spp. Najas marina Chara spp. Terrest. vegetation Phragmites australis Fig. 2. Schematic model of the macrophyte succession in the morphometric development of shallow bays with the land uplift process in the Baltic Sea. Left arrow indicate direction of time and isolation from the sea. I. Juvenile flad, II. Flad, III. Glo-flad, IV. Glo. (Based on data in Paper I, pers. obs. and Munsterhjelm 2005). Studied species The flora and fauna of the brackish Baltic Sea consist of a mixture of marine and freshwater species. The flora and fauna diversity and composition change with the latitudinal gradient of the Baltic Sea. The diversity of marine algae and macroinvertebrates decreases from south to north, while the diversity of freshwater algae, angiosperms and macroinvertebrates increases (e.g. Zenkewich 1963, Nielsen et al. 1995, Snoeijs 1999). The composition of marine and freshwater species also changes with distance from the main land as coastal areas near land generally have a lower salinity than areas further out towards the open sea (e.g. Wallentinus 1976). Among the most common macrophytes in shallow soft-bottom bays in the studied geographic area are P. pectinatus, M. spicatum and Chara baltica Bruzelius (Paper I; Wallström & Persson 1997, Edlund & Siljeholm 2003, Johansson & Persson 2005) (Table 1). Potamogeton pectinatus and M. spicatum are freshwater angiosperms (Preston & Croft 2001, Mossberg & 11

12 Stenberg 2003), while C. baltica is a brackish-water charophyte (Schubert & Blindow 2003). These three species co-occur more or less in all geomorphological developmental stages of bays, except in very isolated bays (Munsterhjelm 1997; pers. obs.; Johansson G. pers. comm.). They grow in patches together with other submerged macrophytes, or form singlespecies meadows that can cover large areas within the bays. They are perennials; however the biomass of shoots during winter is low. Potamogeton pectinatus, M. spicatum and C. baltica were used in the second study included in the thesis, where the habitat preferences of macroinvertebrates were investigated (Paper II). Table 1. Life history, geographic range and morphological characteristics of the three macrophyte species used for the experiments in Paper II. Reproduction Macrophyte species Overwinter (Fertile) Tuber cd Potamogeton pectinatus June-Aug a Shoot Geographic Leaf Leaf Max Branching length with height distribution 1 (cm) (mm) (no./mg d.w. * ) (m) BP, BS, BB a 3 11 a a 2.2 f 2.5 fg Myriophyllum spicatum June-Aug a Shoot d BP, BS a 2 3 a <0.5 f 16.1 f fg Chara baltica June-Nov b Bulbill be Shoot BP, BS, BB b 1-8 f <1 bf 1.1 f 1.5 b 1 BP = Baltic Proper, BS = Bothnian Sea, BB = Bothnian Bay; * dry weight a Mossberg & Stenberg 2003; b Schubert & Blindow 2003; c Idestam-Almqvist 1998; d Kautsky 1990; e Wallentinus 1979; f pers. obs.; g G. Johansson pers. comm. Four common invertebrate taxa in shallow soft-bottom bays were used in the habitat preference experiment (density data from Paper I); one amphipod genus (Gammarus spp. [mainly G. oceanicus Segerstråle], one isopod genus (Idotea spp. [mainly I. chelipes (Pallas)]) and two gastropod species (Theodoxus fluviatilis (L.) and Bithynia tentaculata (L.)) (Table 2). There are five Gammarus species in the Baltic Sea; G. oceanicus, G. salinus Spooner, G. locusta (L.), G. zaddachi Sexton and G. duebeni Liljeborg. All are of marine origin but are distributed through the whole Baltic Sea and co-occur at the same salinities (Fenchel & Kolding 1979, Foberg 1994). The Gammarus species show a habitat selection in accordance with wave exposure (Fenchel & Kolding 1979). At sheltered sites in the northern Baltic Proper G. oceanicus dominate during most time of the year (Haage 1975, Fenchel & Kolding 12

13 1979). G. oceanicus reproduce from mid summer to early autumn, whereafter large individuals die off (Haage 1976, Kolding & Fenchel 1979) (Table 2). Among the most common isopods in the Baltic Sea are the three Idotea species I. baltica (Pallas), I. chelipes (Pallas) and I. granulosa Rathke. Of these I. chelipes and I. baltica are most tolerant to low salinities and I. chelipes is mainly found in shallow, sheltered environments (Salemaa 1979). I. chelipes is distributed up to Bothnian Bay, while I. baltica is distributed up to the Northern Quark (Foberg 1994) (Table 2). In the Baltic Sea the time for reproduction of Idotea has been documented from May to August (Haage 1976, Salemaa 1979, Jormalainen & Tuomi 1989). During this period the adults reach their largest bodysize, while the population density decrease as adults dies after reproduction (Salemaa 1979). The highest population densities of Idotea have been recorded in autumn (October to December) as a result of the new cohort from the summer reproduction (Haage 1976, Salemaa 1979). Theodoxus fluviatilis and B. tentaculata are freshwater prosobranchs (Orthogastropoda). Theodoxus fluviatilis is very common and abundant on rocky littoral shores throughout the Baltic Sea (Hubendick 1949, Skoog 1971, Foberg 1994), while B. tentaculata is distributed only in coastal areas with low salinities (Hubendick 1949, Foberg 1994). Egg capsules of T. fluviatilis have been observed during the whole year in the northern Baltic proper, but most juveniles hatch during summer (Haage 1976; Table 2). The population densities of T. fluviatilis are highest during autumn (Haage 1975). In European freshwaters juvenile B. tentaculata generally hatch during summer and the population densities are highest in late summer (Dussart 1979; Table 2, Richter 2001). Categorization of fauna into functional groups Organisms can be classified according to different ecological traits. Animals can be classified according to factors such as habitat occupation, trophic level or way of feeding. Merritt & Cummins (1984, 2006) developed a classification system of macroinvertebrates into functional feeding groups based on the morphology and behaviour of the animals food acquisition. This classification is suggested to be advantageous compared to classifications according to trophic level (herbivore, carnivore and detritivores) for aquatic macroinvertebrates as a vast majority of these species are more or less omnivores (Merritt & 13

14 Cummins 1984). In the functional feeding group classification macroinvertebrates are categorized as scrapers - feeding on periphyton; shredders - chewing and mining live and decomposing plant material; filtering collectors - filter or suspension feeds on fine particulate matter; gathering collectors - gather or deposit feeds on fine particulate matter; piercers - suck contents of plant cells; and predators - feeds on animal tissue. The functional feeding group classification enables assessments of which food sources the invertebrate community of a given aquatic system is dependent upon. In the present study I categorize the fauna taxa into functional groups using the functional feeding group classification by Merritt & Cummins (1984, 2006) (Table 2 and Appendix in Paper I). Table 2. Life history characteristics, geographic range and categorisation into functional feeding groups of the macroinvertebrate fauna taxa used for the experiments in Paper II. Macrofauna taxon Density peak Reproduction Generations per year Max life length Geographic distribution 1 Functional group Gammarus spp. Autumn-winter a Whole year a 1 2 a --- BP, BS, BB bc Shredders defg G. oceanicus --- June-July ah 1 ah 1 year a BP, BS, BB bc Shredder ef Idotea spp. Oct-Dec ak May-July akl 1-2 a year k --- Shredders efijy I. baltica Oct-Dec ak May-July akl year k BP, BS c Shredder efi I. chelipes Oct-Dec ak May-July ak 1 2 a 1 year k BP, BS, BB c Shredder ijy Theodoxus fluviatilis Autumn q Mainly summer a 1 2 a 3 years rs BP, BS, BB cmn Scraper op Bithynia tentaculata Late summer wx Summer wx years x BP, BS, BB cn Scraper, filtering collector tuv 1 BP = Baltic Proper, BS = Bothnian Sea, BB = Bothnian bay; a Haage 1976; b Fenchel & Kolding 1979; c Foberg 1994; d MacNeil 1997; e Goecker & Kåll 2003; f Kotta et al. 2004; g Cummins & Clug 1979; h Kolding & Fenchel 1979; i Salemaa 1987; j Verhoeven 1980; k Salemaa 1979; l Jormalainen & Tuomi 1989; m Skoog 1971; n Hubendick 1949; o Neumann 1961; p Skoog 1978b; q Haage 1975; r Skoog 1978a; s Kirkegaard 2006; t Brendelberger 1997; u Brendelberger & Jürgens 1993; v Brendelberger 1995; w Dussart 1979; x Richter 2001; y Sommer

15 Field survey (Paper I) Samples of submerged aquatic macrophytes and macroinvertebrate fauna were taken by freediving in the 12 bays chosen for sampling. In each bay, 9 samples were collected along 3 transects located in the inner, middle, and outer parts of the bay. I measured and estimated several morphometric characteristics of the bays, all related to the isomorphic land-uplift development. The abiotic factors measured and estimated were water exchange, depth, wave exposure and salinity of the bays. As biotic factors I recorded the average (per sample) biomass and number of macrophyte taxa in the bays (mean macrophyte taxon richness), as well as the sum of all flora taxa found in all 9 samples from a bay (total macrophyte taxon richness). For the analyses of the relationship between univariate measures of the fauna community and the environmental variables I chose to focus on water exchange of the bays and the mean number of macrophyte taxa in the bays. The first parameter was well correlated with several of the other abiotic factors and the latter is a robust biotic measure that can be obtained using several survey methods, such as collecting plant biomass material and visually observing plant cover. The univariate measures of the fauna community recorded were mean taxon richness (per sample), total taxon richness (sum from all samples in a bay) and mean biomass (per sample). The relationship between these measures of the fauna community and the environmental variables water exchange and mean macrophyte taxon richness were analysed by structure equation modelling (SEM). In SEMs, as opposed to multiple regressions, variables can be both responses and predictors in the same model. This provides opportunities to test hierarchical relationships between paths in a specific model. I tested if the effect of water exchange on the fauna community was indirect rather than direct, i.e. if there was a hierarchical relationship through a water-exchange flora fauna path rather than a direct water-exchange fauna relationship. Additionally, I tested how well the environmental variables water exchange and macrophyte taxon richness explained the variation in the multivariate community composition of fauna, and the variation in the relative proportions of functional feeding groups in relation to the total biomass of the fauna, using canonical redundancy analyses (RDA). 15

16 Differences in community composition of animals between specific macrophyte species could not be conducted as the plant species were unevenly distributed between the morphometric isolation stages of bays and between regions. Experimental set-up (Paper II) In order to examine potential differences in community composition of animals between macrophyte species I conducted a habitat preference experiment. The experiment allowed studies of short time habitat choice of macroinvertebrates between macrophyte habitats in a controlled abiotic environment. I examined the active habitat choice of four fauna taxa, Gammarus spp., Idotea spp., T. fluviatilis and B. tentaculata, between the three macrophytes C. baltica, M. spicatum and P. pectinatus. Additionally, I studied the animals habitat preferences between the single-species habitat and a three-species habitat consisting of a mixture of the three species. The habitats were compared pair-wised, resulting in 6 combinations. These were replicated 9 times and the differences between the habitats were analysed by means of binominal goodness-of fit tests To identify factors responsible for the habitat preference I quantified differences in morphology and epiphyte density between the three macrophyte species. Morphology was measured on 17 macrophytes taken from the habitat experiment and collected in the field. For the epiphyte examination 5 replicates of the three macrophyte species were taken in the field. I also conducted a feeding preference study with one of the herbivores (Idotea) in order to examine if the crustacean had any preference in feeding between the three selected macrophytes and an additional epiphytic green algae; Cladophora glomerata (L.) Kützing. The feeding rate was compared in pair-wise sets of the macrophytes, resulting in 6 combinations. These were replicated 10 times and differences in biomass change of the macrophytes were tested with Wilcoxon matched pair tests. 16

17 Results and discussion Paper I With the land-uplift process operative in northern Europe, shallow coastal bays in the Baltic Sea are decreasing in e.g. depth and water exchange. In this study I demonstrated that as a result of the morphometric development, the macroinvertebrate fauna biomass and taxon richness also decrease (Fig. 2). The two environmental variables chosen as predictors for the macroinvertebrate fauna community (water exchange and mean macrophyte taxon richness) proved to explain much of the variation in the fauna assemblages. The path analyses in the SEMs demonstrated that 78 88% of the variation in the univariate measures of the fauna community was explained by the two included predictor variables (Fig. 3). For all three univariate measures of the fauna community (biomass, total taxon richness, and mean taxon richness) there was a hierarchical relationship between the abiotic factor water exchange, the macrophyte community (mean taxon richness), and the response variable. Decreasing biomass and taxon richness of fauna were related to decreasing macrophyte taxon richness, which in turn was related to decreasing water exchange. A similar decrease in macrophyte taxon richness was found by Munsterhjelm (1997) in his study of the flora community in similar shallow bays along the Finnish coast of the Baltic Proper. The multivariate analyses of the fauna composition also demonstrated that the community changed with the morphometric isolation gradient of the bays (Table 3 and Fig. 4 in Paper I). The variation in the multivariate community composition of the fauna could to a large degree be explained by the two environmental variables included in the analyses. The two axes derived in the RDAs could explain 47.7% and 57.5% of the variation in taxon composition and relative proportion of functional fauna groups respectively (Table 3 in Paper I). The community changed from a dominance of gastropods and bivalves in the sounds and most open bays to a dominance of mainly a few insect taxa in bays with very narrow and shallow openings (Fig. 4). Similar to the univariate analysis the multivariate RDA demonstrated that the mean macrophyte taxon richness explained more of the variation in the fauna community than did water exchange. The relative proportions of functional groups of fauna changed in a similar manner, and the community shifted along the bay isolation gradient from one dominated by scrapers and filtering collectors (mainly gastropods and bivalves) to one dominated by gathering collectors (several insect taxa), predators (mainly insect taxa), and 17

18 shredders (crustaceans and insects) (Fig. 4D in Paper I). Hence, the macroinvertebrate community shifted from a dominance of taxa dependent on small particulate matter or microscopic algae (planktonic or epiphytic) as food, to a community dependent on larger organic particles on the benthos, living plants or animal tissue as food resources. The studies by Munsterhjelm (1997) and Appelgren & Mattila (2005) of the flora community in similar soft-bottom bays also demonstrated a change in the organism composition with the morphometric development. Similarly, M. Snickars & A. Sandström (pers. comm.) observed the community composition of young-of-the-year fish to change with the isolation gradient of shallow soft-bottom bays from the sea in the northern Baltic Proper. Log fauna biomass (g dw*m -2 ) I. II. III Fauna richness (no. of taxa) Log water exchange Fig. 2. Changes in fauna biomass (, solid line), total taxon richness (, dotted line) and mean taxon richness (+, dashed line) along the morphometric isolation gradient of bays from the sea. Bays are separated into groups of four according to water exchange; I. high water exchange (4 open bays and juvenile flads), II. intermediate water exchange (4 flads), III. low water exchange (1 flad and 3 gloflads). (Data from Paper I). 18

19 BM, TS, S = 0.45 Mean macrophyte richness BM = 0.80*** TS = 0.64** S = 0.72*** BM, TS, S = 0.74*** Fauna community a,b,c BM = 0.17 TS = 0.22 S = 0.12 Water exchange BM = TS = S = Fig. 3. Path diagram of water exchange, mean macrophyte taxon richness, and univariate fauna community variables; biomass (BM), total taxon richness (TS), and mean taxon richness (S). Numbers are individual standardized path coefficients with superscripts indicating significance (*p < 0.05, **p < 0.01, ***p < 0.001). (From Paper I). A 1.0 B Ca.spp Ma.bal Relative density Relative density Po.ant Th.flu Bi.ten Ra.bal My.edu I II III Gastropoda Bivalvia Crustacea Insecta Relative density Ga.spp Pa.ads Id.che Ba.imp I II III I II III Anis Lepto Pyra Ephe Fig 4. (A) Relative distribution of fauna taxonomic groups according to biomass along the morphometric isolation gradient of bays from the sea. Data from the bays were pooled into three groups according to water exchange; I. high water exchange (4 open bays and juvenile flads), II. intermediate water exchange (4 flads), III. low water exchange (1 flad and 3 glo-flads). (B) Relative biomass of 15 taxa representing each of the taxonomic groups in (A); Gastropoda: Potamopyrgus antipodarum, Theodoxus fluviatilis, Bithynia tentaculata, Radix baltica; Bivalvia: Cardium spp., Macoma baltica, Mytilus edulis; Crustacea: Gammarus spp., Palaemon adspersus, Idotea chelipes, Balanus improvisus; Insecta: Anisoptera, Leptoceridae, Pyralidae, Ephemeroptera. Biomass is relative to the highest biomass in one of the water exchange categories for each fauna taxa. (Data from Paper I). 19

20 The close relationship between macrophyte taxon richness and macroinvertebrate composition I found (Fig. 3, Fig. 4 and Table 3 in Paper I) may be an effect of a similar response on the part of the flora and fauna communities to abiotic conditions. It is possible that the macrophyte taxon richness also incorporates other factors, e.g. salinity, ice cover and substrate, which affect the fauna but are not described by water exchange alone. Some of these factors may be related to the latitudinal differences and distance from main land between the bays. However, I cannot exclude that the relationship may be due to a direct effect of the macrophytes on the macroinvertebrate community. This could be a result of increasing habitat variability by the addition of macrophyte taxa. In previous studies, macroinvertebrate abundance and taxon richness have been observed to increase with increased macrophyte diversity or functional diversity (Brown et al. 1988, Parker et al. 2001, Bazzanti et al. 2003). Similarly, fish diversity has been shown to increase with increased macrophyte diversity (Tonn & Magnuson 1982). Paper II Three of the four macroinvertebrate taxa studied displayed an active habitat preference. This preference differed between the taxa. The amphipod Gammarus preferred the M. spicatum habitat over the C. baltica and P. pectinatus habitats (Fig. 5). Between the latter habitats C. baltica was preferred. Also the mixed habitat (containing M. spicatum) was preferred over the single-species habitats of P. pectinatus and C. baltica, but not over M. spicatum. The habitat choice of the isopod taxon Idotea was similar to that of Gammarus, however only significant in one combination. The gastropods T. fluviatilis preferred the P. pectinatus habitat over the M. spicatum habitat, and the mixed habitat over the C. baltica habitat. Theodoxus fluviatilis showed a weak tendency of generally preferring P. pectinatus (also included in the mixed habitat) over all the other habitats (not significant). B. tentaculata did not make any active choice between the four habitats (Fig. 5). This species dug into the sand and stayed there during the experiment. This may have been a response induced by our handling of the animals. Concerning the specific role of macrophyte diversity on the habitat preference of macroinvertebrates I found no significant support that they prefer more species rich habitats. 20

21 There was no consistent pattern of preference for the three-species habitat over all singlespecies habitats by any of the macroinvertebrate taxa (Fig. 5). When a three-species habitat was preferred this could be explained by the presence of the preferred species within this mixed habitat; i.e. Gammarus spp. preferred the three-species habitat over the P. pectinatus and C. baltica habitats, but not over the M. spicatum habitat. A more variable and species diverse habitat could potentially be advantageous if the fauna use different macrophyte species (microhabitats) for foraging and hiding from predators. In my study the distance between the pair of habitats might have been too short to get such an effect on the habitat choice of the highly motile crustaceans. Although I did not observe a preference for the more diverse habitat on a taxon level, this does not exclude the importance of macrophyte diversity for fauna communities. As the habitat preference differed between the taxa it instead indicates the importance of habitat diversity for the fauna community composition and diversity. Gammarus spp. Idotea spp ** *** *** *** *** n.s n.s. *** n.s. n.s. n.s. Distribution 0.5 Distribution Po.pec Po.pec Ch.bal Ch.bal Po.pec Po.pec My.spi My.spi Ch.bal Ch.bal My.spi My.spi Po.pec Po.pec Mixture Mixture Ch.bal Ch.bal Mixture Mixture My.spi My.spi Mixture Mixture Po.pec Po.pec Ch.bal Po.pec My.spi Ch.bal My.spi Po.pec Mixture Ch.bal Mixture My.spi Mixture Theodoxus fluviatilis Bithynia tentaculata n.s. * n.s. n.s. * n.s. n.s. n.s. n.s. n.s. n.s. n.s. Distribution 0.5 Distribution Po.pec Ch.bal Po.pec My.spi Ch.bal My.spi Po.pec Mixture Ch.bal Mixture My.spi Mixture Po.pec Ch.bal Po.pec My.spi Ch.bal My.spi Po.pec Mixture Ch.bal Mixture My.spi Mixture Fig 5. Habitat preference of Gammarus spp., Idotea spp., Theodoxus fluviatilis and Bithynia tentaculata between three species of macrophytes and an equal mixture of the three macrophyte species; Chara baltica, Myriophyllum spicatum and Potamogeton pectinatus. The animals were allowed to move freely between the habitats for 24 h. Bars show mean percentage distribution of individuals (±95% CI) between the macrophyte habitats. Significance according to; *** p<0.001, ** p<0.01, * p<0.05, n.s. p>0.1. (From Paper II). The three macrophyte species included in the experiment differed both in morphology and in density of epiphytes (Table 1 and Table 2 in Paper II). Therefore I cannot separate which 21

22 factor was most important for the choice of the animals. However, two factors indicate that food availability or quality was not responsible for the preference of M. spicatum by the crustaceans. First, the density of epiphytic microalgae (which was not selectively removed before the experiments) was not higher on M. spicatum than on P. pectinatus and C. baltica (Table 2 in Paper II). Second, Idotea did not consume more of M. spicatum than of the other macrophytes (Fig. 3 in Paper II). Instead it is likely that the morphological structure was responsible for the choice, with the crustaceans preferring a habitat which was delicately branched and had a large shoot surface area. For T. fluviatilis it is more difficult to separate the effects of morphology and density of epiphytes. The habitat preference could be caused by the higher density of microepiphytes on P. pectinatus compared to the densities on the two other macrophytes (Table 2 in Paper II). But I cannot exclude that the morphology of P. pectinatus is a responsible factor for the behaviour of T. fluviatilis. Conclusion The macroinvertebrate fauna community in shallow soft-bottom bays of the Baltic Sea decrease in species number and change in composition with the natural morphometric development stages of the bays. This change can be explained to a large degree by the water exchange and vegetation in the bays. The mean macrophyte taxon richness proved to be a better predictor of the fauna community than water exchange, either because of a direct florafauna relationship or because this factor incorporates other environmental variables affecting the fauna community, e.g. wave exposure. Within bays, under similar abiotic conditions, we can expect the densities of macroinvertebrate species to differ between patches of different vegetation types as a result of active habitat preferences among the animals. I did not find any habitat preference among the tested invertebrates for more species-rich macrophyte habitats, but as the fauna taxa differed in habitat preference we should expect diverse macrophyte habitats to house a more diverse fauna community. The present findings indicate that macrophyte taxon richness can serve as an indicator for the fauna community. However, more research is needed before applying the findings in any generalized way or making any proposals as to how an indicator should be constructed. The 22

23 study also implies that conservation management must be dynamic and consider a natural decrease of biomass and diversity of fauna in shallow bays over the long term (i.e. decades to centuries). This means that for long term protection of shallow bays, protected areas should be constructed to incorporate coastal areas which will develop into shallow bays with time. Future studies My plan is to further investigate how the ecosystem of shallow soft-bottom bays change with the land-uplift process by conducting stable isotope analyses on different trophic levels in the food web; macroscopic and microscopic primary producers, zooplankton, macroinvertebrates and fish. Since the field study showed that the fauna composition change with the isolation of bays from the sea (filterers and scrapers decrease and shredders, gatherers and predators increase) I want to investigate if the number of trophic levels also changes with this gradient. Moreover, I want to study if the inflow of marine derived carbon to the bays decreases and the flow of terrestrial carbon increases with the isolation process of the bays. To examine the role of macrophytes structural complexity for the distribution of macroinvertebrates in more detail I am planning to conduct fauna habitat preference experiments with artificial plants which differ in structural complexity, i.e. plant shoot perimeter and branching, but with equal surface area. Additionally, I want to study colonization of plants with different structural complexity in the field. In a collaboration project with Upplandsstiftelsen and the Swedish Environmental Protection Agency I will identify factors responsible for differences in vegetation cover between bays within the EU Nature 2000 habitat coastal lagoons along the Swedish coast. Within the project I will also try to identify factors responsible for inter-annual variations in the vegetation cover within the bays. The dataset is a compilation of four years monitoring data ( ) on vegetation cover in 35 coastal lagoon habitats along the Swedish coast from the northern Bothnian Sea on the east coast to the southern Kattegatt on the west coast. The study will provide an opportunity to further test the role of different abiotic factors in structuring the ecosystem of shallow soft-bottom bays in the Baltic Sea. 23

24 Acknowledgements First of all I would like to acknowledge my supervisors Lena Kautsky and Sofia Wikström for all support and guidance. I would further like to thank all my colleagues at the Section for Plant Ecology for inspiring company, nice coffee-breaks, and course/holiday trips to Spain. I would like to send a special gratitude to D. Vanhoenacker and J. Dahlgren for help with the statistics, and C. Holeton for linguistic advice. Jag vill även tacka pappa som ställt upp som assistent både 2005 och 2006, och mamma som planerat sin semester därefter. Utan din hjälp pappa, hade inte fältinsamlingen 2005 funkat! Jag vill även tacka B. Gustafsson på Länsstyrelsen i Östergötland som hjälpte till att fixa med boende och båt vid Licknevarpefjärden. Tack, Hanna Axemar för att arbetet på Askö 2006 blev så trevligt och lyckat. Det var bra att du tjatade att vi skulle ha grillparty och äta också! Ett stort tack även till personalen på Askö för all ovärderlig hjälp samt trevliga fester. I would further like to thank M. Hjelm, G. Johansson and J. Persson for inspiring discussions on the ecology of habitat 1150 LAGOONS ;O), and S. Dahlgren for fist introducing me to the aquatic plants in this habitat. The following people are also acknowledged; S. Govella, S. Hallén, J. Honkankangas, H. Lind, S. Lundberg and C. Rasa. This project was supported financially by grants from the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS; to L. K.) and the Stockholm Marine Research Centre (SMRC; to J. H.). 24

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